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The present invention relates to ink jet printing, and in particular to an improved deflection electrode assembly for use in a continuous ink jet printer.
Continuous ink jet printers are well known in the field of industrial coding and marking, and are widely used for printing information, such as expiry dates, on various types of substrate passing the printer on production lines. As shown in FIG. 1, a jet of ink is broken up into a regular stream of uniform ink drops by an oscillating piezoelectric element. The drops then pass a charging electrode where the individual drops are charged to selected voltages. Next, the drops pass through a transverse electric field (deflection field) provided between a pair of deflection electrodes. Each drop is deflected by an amount that depends on its respective charge. If the drop is uncharged, it will pass through the deflection electrodes without deflection. Uncharged and slightly charged drops are collected in a catcher and returned to the ink supply for reuse. A drop following a trajectory that misses the catcher will impinge on the substrate at a point determined by the charge on the drop. Often, each charged drop is interspersed by a guard drop with substantially no charge to decrease electrostatic and aerodynamic interaction between charged drops. As the substrate is moving past the printer, the placement of the drop on the substrate in the direction of motion of the substrate will have a component determined by the time at which the drop is released. The direction of motion of the substrate will hereinafter be referred to as the horizontal direction, and the direction perpendicular to this, in the plane of the substrate, will hereinafter be referred to as the vertical direction. These directions are unrelated to the orientation of the substrate and printer in space. If the drops are deflected vertically, the placement of a drop in the vertical and horizontal direction is determined both by the charge on the drop and the position of the substrate.
Certain inks, including pigmented inks, have a tendency to create micro-satellite drops which are typically 2 to 3 orders of magnitude smaller than the main ink drops. These micro-satellite drops, when passing through the deflection field, tend to move much faster towards the high voltage deflection electrode due to their relatively large charge-to-mass ratio. As a result, these micro-satellite drops often land on the deflection electrode, causing a rapid accumulation of ink on the deflection electrode. As ink accumulates on the deflection electrode, the strength of the deflection field is reduced, resulting in a reduction in print quality. As a result, printer operation must be interrupted to clean the ink buildup off of the deflection electrode.
Certain aspects of a specific embodiment of the present invention relate to a deflection electrode assembly for use in a continuous ink jet printer of the type which projects a stream of ink drops toward a substrate and controls placement of the ink drops on the substrate by selectively charging the individual ink drops and passing the charged ink drops through an electric field created between a pair of opposed deflection electrodes. At least one of the deflection electrodes includes an opening aligned with the ink drop stream so that micro-satellite ink drops can pass through the opening. For example, when negatively charged drops are passed between opposed high and low voltage deflection electrodes, the opening may be provided in the high voltage deflection electrode. In this respect, the high voltage deflection electrode may include first and second longitudinally extending legs positioned adjacent the ink drop stream, opposite the low voltage electrode. The legs define the opening, which is aligned with the ink drop stream. The opening has an open end facing away from the substrate and a closed end facing towards the substrate. The opening may be in the form of a generally rectangular slot which extends longitudinally along the ink drop stream.
A dielectric insulating material may be disposed on the high voltage electrode. The insulating material may include sleeves that slide onto the first and second legs.
The invention may include means for collecting micro-satellite ink drops that pass through the longitudinal opening. The means may comprise absorbing material positioned above the longitudinal opening. Alternatively, the means may comprise a vacuum assembly for collecting the micro-satellite ink drops. Alternatively, the means may comprise an open space above the longitudinal opening. Means, such as a fan or source of pressurized air, may be provided for circulating air through the open space to disperse the micro-satellite ink drops as they pass up through the longitudinal opening.
Another aspect of an embodiment of the present invention relates to a method for reducing ink accumulation on the deflection electrodes in a continuous ink jet printer. The ink jet printer is of the type which projects a stream of ink drops toward a substrate and controls placement of the ink drops on the substrate by selectively charging the individual ink drops and passing the charged ink drops through an electric field created between opposed deflection electrodes. The method includes providing an opening in at least one of the deflection electrodes and aligning the opening with the drop stream so that micro-satellite ink drops can pass through the opening. For example, when negatively charged drops are passed between opposed high and low voltage deflection electrodes, the opening may be provided in the high voltage deflection electrode. The opening may be a generally rectangular slot, which extends longitudinally along the ink drop stream.
The method may also include disposing insulating material on a high voltage electrode to reduce arcing between the high and low voltage electrodes. The method may further include collecting the micro-satellite ink drops that pass through the longitudinal opening. The drops may, for example, be collected by an absorbing material positioned adjacent the longitudinal opening. Alternatively, the micro-satellite ink drops may be vacuumed up as they pass through the opening. Alternatively, air can be circulated through the open space above the opening to disperse the micro-satellite drops as they enter the open space.